1. Field of the Invention
The present invention generally relates to the art of microelectronic integrated circuit fabrication, and more specifically to a method for detecting an adjustment error in a photolithographic stepping printer.
2. Description of the Related Art
Photolithography is an important technique in the fabrication of microelectronic integrated circuits. An optical mask or reticle is prepared which defines a pattern for features that are to be formed by oxidation, deposition, ion implantation, etching, and other techniques. A photoresist layer is formed on a semiconductor wafer on which the features are to be formed. The photoresist layer is optically exposed through the mask or reticle and causes a chemical reaction in areas which are not covered by opaque areas of the mask. The wafer is then developed to selectively remove areas of the photoresist layer.
In a positive photoresist process, the developer dissolves areas of the photoresist layer that were subjected to the optical exposure, leaving the areas that were protected by the opaque areas of the mask on the surface of the wafer. In a negative photoresist process, the unexposed areas are selectively removed by the developer. The remaining areas of the photoresist shield the underlying areas of the wafer such that they are unaffected by subsequent processing steps, whereas the exposed areas of the wafer can be subjected to oxidation, etc.
Various techniques are available for photolithographic exposure, including contact printing in which the mask or reticle is in contact with the wafer surface, and proximity printing in which the mask is close to, but does not contact the surface. A third technique is projection printing, in which an image of the mask is focussed onto the wafer using an optical imaging system.
Projection printing is especially desirable in that it enables the mask or reticle to be made several times larger than the actual size of the features to be formed on the wafer, thereby increasing the resolution level of the mask. The image as projected onto the wafer is reduced in size by the optical system.
Due to the extremely small tolerances of the optical system, an entire wafer is not exposed at once during projection printing. Instead, adjacent portions of the wafer are exposed sequentially. A first portion of the wafer is exposed, and the optical system is moved to a next position by means of a motor drive, and the next portion of the wafer is exposed. A photolithographic projection printer of this type is called a "stepping printer", "stepping aligner", or simply "stepper" or "aligner".
A plurality of integrated circuit devices are conventionally formed on a single wafer. The wafer is then "diced", or cut into individual "dies" which each include a single device. The dies are then subjected to additional operations, and are packaged into individual integrated circuit chips.
The area of a wafer which is exposed at each step of a stepper is called a "field". For the fabrication of small scale integrated circuits, a field can define more than one device. A field can also define a single device or, for large scale integration, a portion of a single device.
Steppers of the type to which the present invention particularly relates are commercially available from ASML of Tempe, Ariz. These steppers typically have a field of 22.times.22 mm and a resolution limit of 0.2 to 0.3 microns. However, the present invention is not limited to any particular type of printer, but is also applicable to proximity printers, step and scan printers, and any other type of optical stepping projection apparatus.
Stepping printers or aligners for photolithography are subject to very close tolerances in order to resolve submicron features. The optical systems have a number of adjustments, including focus, tilt, field curvature, etc., which must be precisely maintained. If an adjustment is off even slightly, the exposure operation can be so defective that the partially formed devices on the wafer will be unacceptable for further processing.
Following development, an inspection, sometimes referred to as an "after-develop-inspection" or ADI, is performed. The purpose is to ensure that the exposure, baking, development and other steps performed so far have been performed correctly and to the specified tolerance. Mistakes or unacceptable process variations can still be corrected, since the resist process has not yet produced any changes (e.g. through an etch step) to the wafer itself. Thus, any inadequately processed wafers detected through the inspection (known as "rejects") can have their resist stripped and reworked.
The inspection can be performed manually or can be automated. Either way, various feature characteristics on the wafer are examined using an optical microscope, scanning electron microscope or optoelectronic imaging device. The characteristics include linewidth, spacing, contact dimensions and variations of linewidths over fields. Adjustment errors in the stepper including focus, tilt and field curvature will result in the characteristics being out of tolerance and are detected in the inspection.
The prior art inspection methods are complicated, time consuming, and require elaborate and expensive equipment. In addition, it is difficult to associate the raw data obtained from the inspection with particular adjustment errors in the stepper. In other words, it is difficult to determine the actual cause of an out of tolerance condition from the data itself.
As such, there exists a need in the art for a simple, fast and inexpensive method for detecting and determining the cause of an adjustment error in a photolithographic stepping printer or aligner.